Dynamic braking has been used in railroad and transit applications to convert braking energy to heat or electrical power, for example by use of one or more electric traction motors of a railroad vehicle for electrical power generation when braking. Such electro-dynamic braking generally significantly lowers the wear of friction-based braking components. In dynamic braking, resistors can be provided to dissipate braking energy, typically by engaging one or more resistors within a bank (a collection of resistors) or one or more entire banks of resistors. In the absence of equipment to store and then convert the resulting heat back to electricity to power the train when later accelerating, such systems typically utilize forced air cooling to simply discharge the braking energy as heat to the atmosphere.
The size of resistive heating loads found in both light and heavy rail can be considered as representative of the loads that can be controlled by some implementations of the current subject matter. The current subject matter is not limited by load or voltage. However, some implementations can be advantageously applied when a supply voltage is significantly higher than a sum of the forward voltage drops of the controlled rectifier components (at least 10 to 1 ratio) under various design conditions.
Mechanically switched unit resistor load stepping is generally not suitable for cases where the resistive load needs to be switched on or off very rapidly to and from the electrical distribution source. Once activated or deactivated, a mechanical switch or relay generally makes contact one or more times and at an unknown position in the phase of each line, which can lead to significant voltage transient spikes.
High rates of change in power when a load is suddenly connected at or near full power can generate a rapid change in the temperature of the heater or heaters. This temperature change can accelerate the damage done by thermal shock and eventual device failure at one or more mechanically weakest connections or any electrical “hot spot.” The sudden power draw or removal coming from massive load switching can overwhelm and possibly damage the electrical motor/generators providing power to the grid. The harmonics and sub-harmonics produced by such a power change can be significant enough to adversely affect portions of the grid (micro-blackouts or brownouts), particularly in distribution areas electrically “close” to the high power switched load or when the grid transmission equipment is near its load limit.
Existing high power precision load controllers with active electronic components typically utilize Pulse Wave Modulation (PWM) of the rectified input mains. Fixed voltage output PWM controllers have reached a composite efficiency of nearly 98.5% (frequently referred to as switching power supplies) and may be found at a wide range of output voltages and power limits. Variable voltage output PWM controllers can typically operate at about 89% efficiency with many models unable to achieve stable low output voltages, causing a typical lower limit of about 1% to 5% of rated output voltage. If the load to be controlled is on the order of megawatts for high demand applications, even a loss of 2% source power reaching the load can result in very significant control electronics power consumption. In addition to being wasteful, the resulting heat must be removed in continuous operation of the system. For example, a 2% loss of a 1 MW load results in 20 KW that goes to the power control components and that can require large, expensive, and less reliable cooling to accomplish accurate load balancing.
In addition, the high frequency switching of PWM can introduce unwanted harmonics and some amount of phase shift to the input power source (AC Mains) due to the reactive (inductance, capacitance) circuit design that makes PWM possible. These harmonics and phase shifts can be compensated for with a more sophisticated design, but at the expense of even lower output efficiency, in addition to more controller components and complexity.
It can be desirable for high power switching to behave as a controlled load for connection to AC mains. However, mechanical contact switches generally cannot be turned on or off with sufficient speed or accuracy to avoid creating random transient excursions on the grid as the electrical switching occurs. Particularly when significant portions of a regional grid are at or near the limit of available generating capacity, the spikes produced by making or breaking of such large loads in an unsynchronized manner can often be sufficient to trigger one or more power line condition protection circuits of connected facilities and to cause them to disconnect from the grid, even if momentarily, which can in turn trigger even greater transients. This effect can therefore lead to a series of overload failures that quickly overwhelms a power grid's ability to accommodate them in a controlled fashion, and can in some cases even escalate to become a serious brownout or blackout major portions of the grid, as was recently demonstrated by the blackout from Phoenix to San Diego.
Accordingly, the current state of the art for high power precision load control or modulation leaves significant room for improvement. As presented in the accompanying diagrams and descriptions, implementations of the current subject matter can include systems, articles of manufacture, methods, techniques, etc. that can improve upon one or more of the above-noted deficiencies or that can provide one or more other benefits or advantages relative to currently available approaches. In addition the specification begins to address some of the related problems associated with integration of large amounts of renewable wind and solar energy. It directly addresses low base load conditions by enabling the controlled, rapid, efficient and precise variation (the “taking”) of large amounts of power load sufficient to counter-balance the large and relative fast changes in electrical energy production of these resources.
The production of large amounts of wind and/or solar energy is an important goal related to renewable resources and achieving national energy security. To accommodate the difference between the temporal distribution of energy generation from such sources (e.g. most wind energy is generated at night, and solar energy does not match the load curve to varying degrees with seasonal changes), storage of generated energy can be necessary. Such energy storage desirably can include capacity sufficient for periods of regeneration time from one hour upwards depending on the level of reserve desired to balance the load curve. With the addition of recharging components and a large capacity storage unit, dynamic loading can also be used to store and regenerate electrical energy. This energy can advantageously be capable of being taken from the AC mains grid and placed into storage under precise high-power (on the order of a megawatt to more than a gigawatt) control.
As an additional consideration, electrical utilities typically demand that loading of the grid (for example by an electrical storage unit) appear as much as possible to be a purely resistive element with no reactive elements that can cause shifts in the phase between a generation source and a connected load, and that such loads appear to be purely resistive elements throughout the dynamic range of power/energy going into or out of storage. The nullification of reactive loading can result in a power factor at or near 1.0, meaning that little or none of the energy received for storage is reflected by reactive components back onto the grid. Since the storage power load can be variable, the use of fixed reactive compensation is generally precluded. Rather, a storage load must be capable of compensating dynamically to match the amount of power taken from the grid. Storage efficacy can be defined by a combination of factors, including, for example, cost and efficiencies of the conversion of electrical energy into storage media, the storage self-discharge rate, and conversion back from the storage media to the grid. For the example of grid storage and retrieval, implementations of the current subject matter can address approaches suitable for highly efficient conversion of electrical energy into thermal energy, whereupon it may be transferred to storage. The concept of high-efficiency conversion of very large amounts of AC electricity to direct current provided as an input to a load is not limited to storage applications.